This work is concerned with the mechanism of assimilation of iron in bacterial species, mainly Escherichia coli K12. Both components of the iron uptake pathway comprising the high affinity system, namely, siderophores and their membrane receptors, will be investigated. The specific transport systems to be studied in detail will be those involving the siderophores ferric enterobactin and ferrichrome. The iron operon of Escherichia coli will be cloned on bacteriophage lambda and from the DNA of the phage the amino acid sequence of the functional and regulatory proteins of the operon will be deduced. The constitution of certain siderophores containing spermidine, such as agrobactin and related compounds, will be clarified by chemical analyses. The role of siderophores and high affinity iron transport in the formation of the nitrogenase complex of enzymes in N2 fixing species will be explored.